1. Field of the Invention
The present invention relates to a pneumatic tire for heavy load, and more specifically to a pneumatic tire for heavy load whose durability, resistance to a wire fatigue, and resistance to an uneven wear can be improved.
2. Description of the Related Art
In a pneumatic tire for heavy load, especially with a wide base and low aspect ratio (the aspect ratio being equal to or less than 70%, for example), a growth in a radius (a deformation in a tire radial direction) markedly occurs in a vicinity of a shoulder part on both sides of a tread part in a tire width direction, and the radius growth ranges unevenly in the tire width direction. This uneven radius growth causes deterioration in a durability of the tire and a resistance to a wire fatigue of a belt layer provided in an outer circumference of a carcass, and also in a resistance to an uneven wear of a shoulder rib and a shoulder block.
One conventional pneumatic tire for heavy load has a width equivalent to at least 80% of a maximum tire width in the tire width direction and includes at least two belt layers which crosses at an angle between 10 degrees and 45 degrees with respect to the tire circumferential direction and an additional layer which is formed by a reinforcing element provided between the belt layers and essentially parallel to the tire circumferential direction. In such a pneumatic tire for heavy load, the belt layers have a width larger than that of the additional layer by at least 16% of the maximum tire width in the tire width direction and are interlinked, on the way extended from the additional layer in the tire width direction, over a distance equivalent to at least 3.5% of the maximum tire width, and end parts of the belt layers in the tire width direction are separated by a rubber mix as disclosed in PCT National Publication No. 2001-522748, for example.
Another conventional pneumatic tire for heavy load makes an attempt to suppress an amount of the radius growth by specifying, with respect to ends in a minimum width of at least two wire layers which are substantially non-stretchable and arranged in the tire circumferential direction, a position of a circumferentially-formed groove provided between a position at a quarter of a tread part and an edge on the tread surface as disclosed in Japanese Patent Application Laid-Open No. H10-250314, for example.
Still another conventional pneumatic tire for heavy load makes an attempt to equalize an amount of the radius growth ranging in the tire width direction by specifying a carcass line as disclosed in Japanese Patent Application Laid-Open No. H5-185806, for example.
Still another conventional pneumatic tire for heavy load makes an attempt to suppress an amount of the radius growth by setting a total force, per unit width, of a belt layer in the tire circumferential direction as disclosed in Re-Publication No. of PCT application entering national phase 2003-61991, for example.
However, any of the conventional pneumatic tires for heavy load described above do not meet requirements for a durability, a resistance to a wire fatigue, and a resistance to an uneven wear as a pneumatic tire for heavy load with a wide base and low aspect ratio.